A First-in-human Study of the Safety of an Immunosuppressive Antibody (IMP761) in Healthy Volunteers

The goal of this clinical trial is to evaluate the safety and tolerability of single and multiple doses of IMP761 in healthy female and male volunteers aged 18-55 with no history of disease affecting the immune system or recent use of medication with effects on the immune system. The main question it aims to answer is: \- if IMP761 is safe and tolerable as determined by assessing vital signs, emerging (serious) adverse events, electrocardiography, and clinical laboratory tests. Researchers will compare IMP761 to a placebo (a look-alike substance that contains no drug) to see if single and multiple doses of IMP761 are safe and tolerable in healthy volunteers. Part B of the study also investigates the effect of IMP761 on the inhibition of the keyhole limpet haemocyanin (KLH) driven immune response compared with placebo. Participants will: * receive IMP761 or a matching placebo intravenously once in single dose (part A and B) and three times in multiple dose (part C) during a 4 day in clinic stay with 4-8 following visits. * receive KLH challenge * be monitored for up to 103 days after the first dose.